Photolithography, or optical lithography, is a process used in semiconductor device fabrication to transfer a pattern from a photomask (also called reticle) to the surface of a substrate. Crystalline silicon in the form of a wafer can be used as a choice of substrate, although there are several other options including, but not limited to, glass, sapphire, and metal.
Photolithography generally involves a combination of substrate preparation, photoresist application, soft-baking, exposure developing, hard-baking, etching and various other chemical treatments (thinning agents, edge-bead removal) in repeated steps on an initially flat substrate. A cycle of a typical silicon lithography procedure can begin by depositing a layer of conductive metal several nanometers thick on the substrate. A layer of photoresist is applied on top of the metal layer. A transparent plate with opaque areas printed on it, called a photomask or reticle (hereinafter used interchangeably), is placed between a source of illumination and the wafer, selectively exposing parts of the substrate to light. Then the photoresist is developed, in which areas of unhardened photoresist undergo a chemical change, such as, for example, polymerization. After a hard-bake, subsequent chemical treatments remove portions of the conductor under the developed photoresist, and then remove the remaining hardened photoresist, leaving conductor exposed in the pattern of the original photomask.
Conventional photolithography uses a light source with a wavelength in the deep ultraviolet range (DUV), or between 250 nanometers and 193 nanometers (nm) in the exposure developing operation. Generally, the shorter the wavelength, the smaller the feature. For example, wavelengths of 250 nm result in features of about 0.25 micrometers (μm), and wavelengths of 193 nm result in features of about 0.13 μm. Smaller features are desirable for faster and more efficient chips. Recently, the use of extreme ultraviolet light (EUV) has been used to create even smaller features compared to conventional lithography techniques. EUV includes wavelengths in a range from 1 nm to 31 nm. EUV in photolithography can produce patterned features of less than 0.05 μm. EUV photolithography, generally performed at 13 nm, uses a series of mirrors to circumvent the absorptive nature of EUV wavelengths.
Due to the intricate techniques which must be used in EUV lithography, particle contamination has presented a challenge in creating substantially defect-free reticles. In conventional photolithography, a pellicle situated between the light source and the photomask can be used to trap particulate contaminates. A “pellicle” is a transparent membrane stretched over a frame to protect a photomask. In EUV lithography, however, these pellicles are typically not used because they are generally organic-based and highly absorptive. This results in very expensive defect detection and repair techniques, such as thermophoresis, in particular for particles less than 80 nm. “Thermophoresis” is a process by which a reticle is heated, creating a temperature gradient between the air above the reticle and the surface of the reticle. Because photolithography is performed in a vacuum, however, this technique is inadequate to remove particles because there is no air in which to create the temperature gradient. Laser ablation can also be used to repair defects, however, current technology only allows particles of greater than 60 nm to be detected.